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Large Signal Modeling of Inversion-Mode MOS Varactors in VCOs

Large Signal Modeling of Inversion-Mode MOS Varactors in VCOs . MOS-AK Meeting 2009 . 2-3 April 2009 at IHP in Frankfurt (Oder). Overview. Motivation Large Signal Modeling of Varactors in VCOs Alternative Modeling Concept Simulation Results Conclusion. MOS varactor. CV-characteristic.

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Large Signal Modeling of Inversion-Mode MOS Varactors in VCOs

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  1. Large Signal Modeling of Inversion-Mode MOS Varactors in VCOs MOS-AK Meeting 2009 2-3 April 2009 at IHP in Frankfurt (Oder)

  2. Overview • Motivation • Large Signal Modeling of Varactors in VCOs • Alternative Modeling Concept • Simulation Results • Conclusion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  3. MOS varactor CV-characteristic Tuning range Motivation Tail-biased differential LCTank VCO Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  4. D=S=B MOS Varactor Structure and CV-characteristic R. L. Bunch and S. Raman, Large-Signal Analysis of MOS Varaktors in CMOS – Gm LC VCOs • Source-Drain-Bulk are short-circuited and connected to Vtune Disadvantages: Advantages: • Made from standard MOS-cell • Falling and rising edge of the CV- characteristic can be used • Strongly nonlinear tuning characteristic Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  5. Accumulation Mode MOS Varactor Structure and CV-characteristic R. L. Bunch and S. Raman, Large-Signal Analysis of MOS Varaktors in CMOS – Gm LC VCOs • the p+ regions of drain and source are replaced with n+ regions Disadvantages: Advantages: • Wider transition from Cmin to Cmax as inversion mode varactors • Best Cmax / Cmin ratio • Lowest parasitic resistance • Not made from standard MOS-cell • Nonlinear tuning characteristic Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  6. Inversion Mode MOS Varactor Structure and CV-characteristic R. L. Bunch and S. Raman, Large-Signal Analysis of MOS Varaktors in CMOS – Gm LC VCOs • Source-Drain are short-circuited and Bulk is connected to supply voltage (PMOS) or ground (NMOS) Disadvantages: Advantages: • Very sharp transition from Cmin to Cmax • Susceptible to induced substrat noise • Made from standard MOS-cell • Best linearity Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  7. Overview • Motivation • Large Signal Modeling of Varactors in VCOs • Alternative Modeling Concept • Simulation Results • Conclusion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  8. Vtank(t) Vtank(t) Varactors incorporated into VCOs Vtune=1 V VDD=2,5 V Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  9. Small-Signal A=1.0 V A=0.1 V A=0.5 V Capacitance Gate voltage • Expression for C(v(t)) is needed • Averaging is subject for debate • Neglecting the higher harmonics • Amplitude of the output signal of the VCO is needed Large Signal Varactor Modeling after R. L. Bunch R. L. Bunch and S. Raman, Large-Signal Analysis of MOS Varaktors in CMOS – Gm LC VCOs Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  10. Fundamental Ceff Harmonics Large Signal Varactor Modeling after A. Abidi I Oscillating capacitance as Fourier series: Kirchhoff and tank voltage as Fourier series: Complete inductor and capacitor current: Comparing coefficients at every frequency gives: E. Hegazi and A. A. Abidi, Varactor Characteristics, Oscillator Tuning Curves, and AM-FM Conversion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  11. Large Signal Varactor Modeling after A. Abidi II Graphical ansatz to calculate Ceff : Small signal capacitance approximated with a step function: E. Hegazi and A. A. Abidi, Varactor Characteristics, Oscillator Tuning Curves, and AM-FM Conversion • Expression for C(v(t)) is needed • Includes only 1st and 2nd harmonic of the nonlinear varactor characteristic • Includes only the fundamental of the voltage, higher harmonics are neglected • Amplitude of the output signal of the VCO is needed Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  12. Overview • Motivation • Large Signal Modeling of Varactors in VCOs • Alternative Modeling Concept • Simulation Results • Conclusion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  13. Current of the differential pair: Tank resistance: A.Bunomo, „Determining the Oscillation of differential VCOs“, 2003 C(Vt) Equivalent circuit of the inductor: Differential Equation System for a VCO Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  14. Intrinsic Capacitance Model based on EKV Cgs / Cgd Interpolated intrinsic capacitances: Cgb NMOS transistorWidth = 100 µmVtune = 0V Normalized Capacitances Cbs / Cbd Gate Voltage Interpolation function: With: C. Enz, F. Krummenacher and E. Vittoz, ”An Analytical MOS Transistor Model Valid in All Regions of Operation and Dedicated to Low-Voltage and Low-Current Applications”, Analog Integrated Circuits and Signal Processing, Kluwer, 1995 Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  15. 0 0 0 0 Voltage dependant Varactor Capacitance Capacitance NMOS Gate Voltage Capacitance PMOS Gate Voltage Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  16. Simulation Results with IHP SGB25 Technology Capacitance NMOS transistorWidth = 100 µmVtune = 0V Gate Voltage Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  17. Vtank(t) Effective Large Signal Capacitance Assuming complete symmetry between the two MOS-varactors: Complete varactor capacitance is a series connection of two MOSFETs: Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  18. Overview • Motivation • Large Signal Modeling of Varactors in VCOs • Alternative Modeling Concept • Simulation Results • Conclusion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  19. VDD=2.5 V Capacitance Capacitance Vtank(t) NMOS transistorWidth = 250 µmVtune = 0.9V Tank Amplitude Gate Voltage Effective Large Signal Capacitance Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  20. VDD=2.5 V Capacitance Capacitance Vtank(t) NMOS transistorWidth = 250 µmVtune = 1.5V Tank Amplitude Gate Voltage Effective Large Signal Capacitance Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  21. Effective Large Signal Capacitance Vtune= 0.2 V Vtune= 0.6 V Vtune= 0.4 V Vtune= 0.8 V Vtune= 2.0 V Vtune= 1.6 V Vtune= 1.0 V Capacitance Capacitance Vtune= 1.8 V Vtune= 1.4 V Tank Amplitude Tank Amplitude NMOS transistorWidth = 250 µm Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  22. Dimensioning Varactors in the VCO Design Process Tank amplitude [V] Time [ns] Design of a 2.4 GHz LC Tank VCO with 20 percent tuning range Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  23. Overview • Motivation • Large Signal Modeling of Varactors in VCOs • Alternative Modeling Concept • Simulation Results • Conclusion Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  24. Conclusion • An implementation of an analytical small signal capacitance model for inversion mode MOS varactors based on the EKV model was presented • Simulation results for the small signal capacitance are in good accordance to simulation results that were obtained by using Spectre simulator • If the varactors are incorporated into a VCO a large signal analysis of the varactor capacitance is needed • Two well-established large signal varactor capacitance modeling concepts have been presented and analyzed • An alternative capacitance model in dependency of the output signal of the VCO including higher harmonics was presented • Using this nonlinear modeling approach it is possible to set up a complete nonlinear VCO model that is only dependant of circuit and process parameters Goal: Parameter optimization in advance of the actual design flow Jan BremerLarge Signal Capacitance Modeling 03.04.2009

  25. The End Thank you for your attention! Jan BremerLarge Signal Capacitance Modeling 03.04.2009

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